In the fabrication of various silicon devices such as MNOS field effect transistor devices and the like, the silicon wafer substrate is variously doped to form p-type and n-type regions which are coated with double dielectric layers of silicon oxide, usually thermally grown, and silicon nitride. These regions are connected to each other and to other parts by means of contacts applied to the silicon surface. Thus openings must be formed through the silicon nitride and silicon oxide layers to expose the underlying silicon surface for deposition of the conductive metal contact. This is generally done using conventional photolithographic techniques whereby a layer of photoresist is applied, exposed to a suitably patterned mask and developed to form the desired openings through the photomask to the silicon nitride layer and underlying silicon oxide layer, which layers are then etched away in the opened areas. Optionally, a chemical vapor deposited (CVD) silica protective layer is applied over the silicon nitride layer and this layer also must be etched away.
Several etchants for silicon nitride and silicon oxide are known. Refluxing ortho-phosphoric acid is an excellent etch for silicon nitride, but it does not etch silicon oxide. Aqueous hydrogen fluoride etches both silicon nitride and silicon oxide, but etches silicon oxide much faster than silicon nitride. See Harrap, Semiconductor Silicon, The Electrochemical Society, Princeton, New Jersey (1973) pp. 354-359. Aqueous hydrogen fluoride buffered with ammonium fluoride is a rapid etch (1000 angstroms per minute) for silicon oxide, but etches silicon nitride only very slowly (about 15 angstroms per minute). Thus this etchant requires long processing times for the silicon nitride and also gives a geometry, e.g. undercuts, to the etched walls unfavorable for further processing such as metallization. In order to form slightly tapered walls which are narrower near the silicon substrate, the silicon nitride etch rate should be at least equal to and preferably faster than the silicon oxide etch rate. This configuration of the etched walls insures uniformity and complete metallization of the etched profile.
Thus in order to obtain the desired geometric configuration of the etched pattern, the solid state industry has used a three-step etch whereby the nitride is etched in hot phosphoric acid solution, the oxide is then etched in a separate hydrogen fluoride solution and the overhanging lip of silicon nitride produced during the oxide etch is removed in hot (180.degree. C.) 85 percent phosphoric acid. Careful control of this process is required to avoid under or over etching and to avoid undercutting of the layers by the several etchants. Further, etching over half of the silicon nitride tends to adversely affect that layer's integrity. Certainly a single etchant solution would be preferred to lower the cost of manufacture.
Squillace et al, U.S. Pat. No. 3,811,974, discloses a phosphoric acid-fluoroboric acid mixture for etching silicon nitride-silicon oxide composite structures. By varying the temperature and the concentration of the fluoroboric acid, the relative etch rates may be adjusted. However, etching is slow, on the order of about 100 angstroms per minute, and in practice the rate of etching of the silicon nitride is generally slower than the rate of etching of the silicon oxide.
Thus a single etchant for a silicon nitride-silicon oxide composite structure which etches silicon nitride faster than silicon oxide to produce the desired tapered wall configuration of the etched areas, would be highly desirable.